全文获取类型
收费全文 | 304篇 |
免费 | 3篇 |
国内免费 | 3篇 |
专业分类
化学 | 110篇 |
晶体学 | 1篇 |
力学 | 9篇 |
数学 | 78篇 |
物理学 | 112篇 |
出版年
2020年 | 6篇 |
2013年 | 15篇 |
2012年 | 13篇 |
2011年 | 8篇 |
2010年 | 8篇 |
2009年 | 5篇 |
2008年 | 10篇 |
2007年 | 11篇 |
2006年 | 15篇 |
2005年 | 8篇 |
2004年 | 6篇 |
2003年 | 7篇 |
2002年 | 9篇 |
2001年 | 6篇 |
2000年 | 11篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 5篇 |
1996年 | 8篇 |
1995年 | 5篇 |
1994年 | 8篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 4篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1985年 | 3篇 |
1984年 | 7篇 |
1981年 | 6篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1977年 | 9篇 |
1976年 | 3篇 |
1974年 | 3篇 |
1972年 | 2篇 |
1971年 | 2篇 |
1969年 | 2篇 |
1931年 | 2篇 |
1929年 | 3篇 |
1928年 | 2篇 |
1920年 | 2篇 |
1918年 | 2篇 |
1910年 | 3篇 |
1901年 | 3篇 |
1893年 | 2篇 |
1890年 | 3篇 |
1885年 | 2篇 |
排序方式: 共有310条查询结果,搜索用时 15 毫秒
11.
以内蒙古地区沙棘果实为原料,研究了提取黄色素的工艺条件,同时对该色素的性质进行了初步探讨。结果表明,以95%的乙醇溶液作浸提剂,提取的黄色素浓度最高,工艺流程简单易行,且无毒,无污染。对提取的黄色素进行的性质试验表明,沙棘黄色素对光、热具有较好的稳定性,适用于酸性或弱碱性的食品中,葡萄糖、氧化剂(H2O2)、还原剂(Na2SO3)等食品添加剂均无明显影响以上结果为这种优良天然色素的开发与应用提供了参考。 相似文献
12.
Most p-block metal amides irreversibly react with metal alkoxides when subjected to alcohols, making reversible transformations with OH-substrates a challenging task. Herein, we describe how the combination of a Lewis acidic square-planar-coordinated aluminum(iii) center with metal–ligand cooperativity leverages unconventional reactivity toward protic substrates. Calix[4]pyrrolato aluminate performs OH-bond activation of primary, secondary, and tertiary aliphatic and aromatic alcohols, which can be fully reversed under reduced pressure. The products exhibit a new form of metal–ligand cooperative amphoterism and undergo counterintuitive substitution reactions of a polar covalent Al–O bond by a dative Al–N bond. A comprehensive mechanistic picture of all processes is buttressed by isolation of intermediates, spectroscopy, and computation. This study delineates how structural constraints can invert thermodynamics for seemingly simple addition reactions and invert common trends in bond energies.The combination of structural constraint and metal–ligand cooperativity in calix[4]pyrrolato aluminate inverts common trends of bond energies and enables reversible OH-bond activation. 相似文献
13.
14.
15.
This paper presents a 3D topology optimization approach for designing shell structures with a porous or void interior. It is shown that the resulting structures are significantly more robust towards load perturbations than completely solid structures optimized under the same condi-tions. The study indicates that the potential benefit of using porous structures is higher for lower total volume fractions. Compared to earlier work dealing with 2D topology opti-mization, we found several new effects in 3D problems. Most notably, the opportunity for designing closed shells signifi-cantly improves the performance of porous structures due to the sandwich effect. Furthermore, the paper introduces improved filter boundary conditions to ensure a completely uniform coating thickness at the design domain boundary. 相似文献
16.
Dr. Debangshu Chaudhuri Dr. Eva Sigmund Annemarie Meyer Lisa Röck Philippe Klemm Sebastian Lautenschlager Agnes Schmid Shane R. Yost Prof. Dr. Troy Van Voorhis Dr. Sebastian Bange Prof. Dr. Sigurd Höger Prof. Dr. John M. Lupton 《Angewandte Chemie (International ed. in English)》2013,52(50):13449-13452
17.
18.
Improved contrast to tissue ratio at higher harmonics 总被引:4,自引:0,他引:4
The challenge in ultrasound contrast imaging is a better discrimination between the perfused tissue and the contrast bubbles, which is usually expressed by contrast to tissue ratio (CTR). Imaging based on the second harmonic frequency showed a higher CTR than imaging at the fundamental frequency. However, because of nonlinear propagation of ultrasound waves, harmonic frequencies are generated. These harmonic frequencies will be linearly reflected by the tissue and therefore limit the CTR at the second harmonic frequency. In order to reduce the scattering of tissue at harmonic frequencies and by that increase the CTR, nonlinear distortion has to be reduced. We demonstrate in this study that the CTR increases with the harmonic number. The increase is substantial when transmitting at lower frequencies. To take advantage of the higher harmonics (third, fourth, fifth and the ultraharmonics and termed here super harmonics), we have developed a new phased array transducer with a wide frequency band. In-vitro measurements using the new probe show an increase of 40 dB of the CTR for super harmonic components over the conventional second harmonic system. The increase in CTR is in agreement with the calculations using existing models for the response of encapsulated bubbles and known theory of nonlinear propagation. 相似文献
19.
Schmidt KS Filippov DV Meeuwenoord NJ van Der Marel GA van Boom JH Lippert B Reedijk J 《Angewandte Chemie (International ed. in English)》2000,39(2):375-377
Cross-linking ability is possible with the oligonucleotide-tethered, monofunctional trans-Pt(II) complex shown. It was synthesized by a novel solid-phase approach comprising conjugation of immobilized tetrathymidylic acid with a trans-a(2)Pt(II) building unit, ammonolysis, and transformation of the resulting complex (R=1-N-cyclohexylmethylthyminate) into the chloro derivative (R=Cl). a=NH(2)CH(3), T=thymine. 相似文献
20.